This 7.1.0 website was running +/- 2 years until today it suddenly stopped working and shows this, even if I try to access the umbraco backoffice:

Exception Details: System.Xml.XmlException: Root element is missing.

[XmlException: Root element is missing.] System.Xml.XmlTextReaderImpl.Throw(Exception e) +69 System.Xml.XmlTextReaderImpl.ParseDocumentContent() +5576760 System.Xml.XmlTextReaderImpl.Read() +215 System.Xml.Linq.XDocument.Load(XmlReader reader, LoadOptions options) +44 System.Xml.Linq.XDocument.Load(String uri, LoadOptions options) +59 Umbraco.Core.Services.SectionService.LoadXml(Action1 callback, Boolean saveAfterCallback) +132 Umbraco.Core.Services.SectionService.<GetSections>b__3() +86 Umbraco.Core.Cache.<>c__DisplayClass81.

[HttpException (0x80004005): Root element is missing.] System.Web.HttpApplicationFactory.EnsureAppStartCalledForIntegratedMode(HttpContext context, HttpApplication app) +9859725 System.Web.HttpApplication.RegisterEventSubscriptionsWithIIS(IntPtr appContext, HttpContext context, MethodInfo[] handlers) +118 System.Web.HttpApplication.InitSpecial(HttpApplicationState state, MethodInfo[] handlers, IntPtr appContext, HttpContext context) +172 System.Web.HttpApplicationFactory.GetSpecialApplicationInstance(IntPtr appContext, HttpContext context) +336 System.Web.Hosting.PipelineRuntime.InitializeApplication(IntPtr appContext) +296

[HttpException (0x80004005): Root element is missing.] System.Web.HttpRuntime.FirstRequestInit(HttpContext context) +9873912 System.Web.HttpRuntime.EnsureFirstRequestInit(HttpContext context) +101 System.Web.HttpRuntime.ProcessRequestNotificationPrivate(IIS7WorkerRequest wr, HttpContext context) +254

Any Ideas?